Functionalized vinyl acetate ethylene binders for paper and paperboard coatings

ABSTRACT

A paper product comprising a planar fibrous cellulose substrate, and a coating composition disposed on a surface of said substrate comprising an interpolymer prepared by the emulsion polymerization of (i) one or more vinyl ester monomers; (ii) ethylene; (iii) an unsaturated silane co-monomer; and (iv) one or more emulsion-stabilizing ionic co-monomers comprising one or more of unsaturated, substituted sulfonic acid, one or more of an unsaturated phosphonic acid, or an ester of one of those acids, in the presence of (v) a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material; and (v) water.

FIELD OF THE INVENTION

The present invention relates to aqueous, surfactant-stabilized, functionalized vinyl ester/ethylene-based emulsion interpolymers demonstrating enhanced dry pick strength when used in paper coating formulations.

BACKGROUND OF THE INVENTION

Pigmented paper coating formulations generally comprise an aqueous synthetic polymer binder emulsion and pigment and may contain other additives typically used in the paper coating art. Illustrative of the polymer binders in the emulsion field are vinyl acetate copolymers and interpolymers, including vinyl acetate/ethylene and vinyl acetate/alkyl acrylate copolymers and interpolymers, and styrene/butadiene and styrene/acrylate copolymers. Such copolymers and interpolymers can also contain other co-monomers such as, for example, a copolymerized ethylenically unsaturated mono- or dicarboxylic acid or other unsaturated co-monomers which can function as cross-linking agents.

U.S. Pat. No. 4,395,499, for example, discloses high strength pigment binders for paper coating having increased water retention and stability. The coating compositions contain an aqueous synthetic polymer latex comprising a dispersed interpolymer of a vinyl ester; a polyethylenically unsaturated co-monomer which can be triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl fumarate, divinyl benzene or diallyl phthalate; an ethylenically unsaturated mono- or dicarboxylic acid co-monomer or half ester thereof; and optionally an alkyl acrylate co-monomer.

Notwithstanding the availability of these various types of paper coating binder emulsions, there is a persistent need for coated paper and coated paperbord producers to identify paper coating emulsions (i.e., paper coating binders) which provide, when used in paper coating compositions, increased binding strength when the compositions are applied to paper and paperboard products. Most often, binding strength is quantified by a paper test called the IGT pick resistance test. In the IGT pick test, the higher the IGT value, the stronger the binder and vice versa.

A variety of emulsion polymerization components and techniques can influence binding strength, but, in general, vinyl acetate-based binders (e.g., polyvinyl acetate, vinyl acetate-ethylene, vinyl acetate-acrylate, and the vinyl acetate-based binders discussed above) are known to provide lower binding strength than more commonly used coating binders like styrene butadiene and styrene acrylate. To compensate for the lower IGT pick resistance provided by paper coating compositions using such vinyl ester based binders, higher binder levels are required, and this, of course, hurts the profitability of the coated paper and paperboard products made with these types of coating binders.

Over the years, very few vinyl acetate/ethylene (VAE) copolymers and interpolymers have been developed for paper coating applications because of the issues associated with handling ethylene gas.

U.S. Pat. No. 3,337,482, for example, discloses paper coating compositions containing pigments and binder emulsions comprising copolymers of ethylene, vinyl acetate and ethylenically unsaturated mono- or di-carboxylic acids such as acrylic acid or maleic acid. The binder emulsions of the '482 patent are prepared by emulsion polymerization of the co-monomers using a nonionic emulsfier which contains polyoxyethylene oleyl or lauryl phenyl ethers.

EP 0 140 227 discloses high strength pigment binders for paper coating having increased water retention and stability. The coating compositions comprise an aqueous synthetic polymer latex and pigment and may contain other additives used in the art of pigmented paper coating. The latex comprises a vinyl ester, an organofunctional silane, carboxyl and/or amide functionality and optionally ethylene and/or other polymerizable comonomers.

More recently, however, the equipment and expertise has been developed to polymerize and stabilize environmentally-friendly vinyl ester/ethylene, e.g., vinyl actetate/ethylene (VAE), coating binders very easily on a commercial production scale. Accordingly, the current state of the emulsion polymerization art with respect to surfactant-stabilized, vinyl ester/ethylene copolymers and interpolymers make such polymeric materials very desirable candidates for paper/paperboard coating applications. Such commercial potential for VAE-based products can be realized if the binding strength exhibited by such VAE-type binders can be made comparable to the binding strength of non-vinyl ester-based binders such as those based on styrene butadiene and styrene acrylate.

U.S. Pat. Publ. 2012/0021237 discloses surfactant-stabilized latex emulsions that can be used as binders in paper coating compositions. Such latex emulsions comprise an interpolymer formed by emulsion polymerizing monomers selected from vinyl esters, e.g., vineyl acetate; ethylene; certain unsaturated mono- and di-carboxylic acid materials such as acrylic acid or maleic anhydride; and certain polyethylenically unsaturated cross-linking co-monomers such as diallyl phthalate. These latex emulsions are stabilized with surfactants that are substantially free of environmentally suspect alkyl phenol ethoxylates (APEs). The paper coating compositions containing latex emulsion binders of this type exhibit especially desirable coating strength as quantified by the Dry Pick Values (as defined therein) which such compositions provide.

Conventionally, these copolymers are prepared by polymerizing appropriate co-monomers in an aqueous emulsion. Such emulsions can be stabilized by adding conventional surfactants (anionic, nonionic, cationic) as emuslifiers. Such emulsions may also be stabilized by including protective colloids such as those based on polyvinyl alcohols (PVOH), ionically modified starches, water-soluble starches, starch ethers, polyacrylic acid, carboxymethyl cellulose, natural gums, gelatin, synthetic polymers, or water-soluble cellulose ethers such as hydroxyethyl cellulose (HEC).

Substantially all-surfactant-based vinyl ester/ethylene (VAE) latex emulsions (i.e., those containing very little or no protective colloid as emulsion stabilizers) are especially desirable from the standpoint of permitting effective compounding of the emulsion with the various types of filler materials which are used in paper coating compositions. Substantially all-surfactant stabilized binder emulsions also provide excellent compatibility with other materials typically used by the paper industry in paper manufacture such as styrene-butadiene rubber (SBR) emulsions.

Notwithstanding such filler compounding and compatibilty benefits, substantially all-surfactant-based vinyl ester/ethylene emulsions, when serving as paper coating compositions, can lead to some processing problems during paper manufacture. In particular, such processing problems can manifest themselves when the paper coated with the all-surfactant binder emulsion exits the curing oven at 110° C.-120° C. and then travels over a series of guide rollers and possibly through a shearing machine. Substantially all-surfactant stabilized coating compositions have a tendency to transfer to the rollers and create build-up which can cause maintenance issues. In addition, at times this build-up can transfer back to the paper which can cause undesired abnormalities on the paper surface.

Attempts have been made to retain the benefits of substantially all-surfactant-based latex binders while minimizing the processing problems hereinbefore described. Such attempts, for example, have involved the use of vinyl ester-ethylene based emulsions stabilized with both surfactant emulsifiers and protective colloids such as PVOH. While some of such attempts have been somewhat successful, these mixed emulsifier/colloid systems must be very precisely formulated and even then still have a tendency not to provide all of the benefits which could be realized by the use of either type of stabilizer system alone. Accordingly, there continues to be a need to identify paper coating compositions based on vinyl ester/ethylene copolymer emulsions which exhibit all the benefits of substantially all-surfactant stabilized emulsions but which do not result in an unacceptable incidence of adverse processing problems during paper manufacture.

It has now been found that by selecting certain types and relative amounts of co-monomers and stabilizing surfactants, especially desirable vinyl ester/ethylene type latex binders can be prepared. Such latex binders are desirable because they can be used to provide especially effective and environmentally friendly coating compositions for paper and paperboard.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to a paper product comprising a planar fibrous cellulose substrate, and a pre-coating composition disposed on a surface of said substrate comprising an interpolymer prepared by the emulsion polymerization of (i) one or more vinyl ester monomers; (ii) ethylene; (iii) an unsaturated silane co-monomer; and (iv) one or more emulsion-stabilizing ionic co-monomers comprising one or more of: (a) an unsaturated, substituted organic sulfonic acid or salt thereof, (b) an unsaturated, organic phosphonic acid or salt thereof, (c) an unsaturated, substituted organic sulfonate or sulfate, or (d) an unsaturated, organic phosphonate or phosphate, in the presence of (v) a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material; and (v) water.

Another embodiment of the present invention is directed to a process for forming a coated paper product, comprising (a) coating a planar fibrous cellulose substrate with a coating composition to form a wet coated substrate, wherein the coating composition comprises an interpolymer prepared by the emulsion polymerization of (i) one or more vinyl ester monomers; (ii) ethylene; (iii) an unsaturated silane co-monomer; and (iv) one or more emulsion-stabilizing ionic co-monomers comprising at least an unsaturated, substituted organic sulfonic acid or salt thereof, an unsaturated, organic phosphonic acid or salt thereof, an unsaturated, substituted organic sulfonate or sulfate, or an unsaturated, organic phosphonate or phosphate, in the presence of (vi) a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material; and (vii) water; (b) drying the wet coated substrate to form a dried coated substrate, and (c) calendering the dried coated substrate to form the coated paper product.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph illustrating the relative Dry Pick Resistance between two examples of the present invention and a conventional paper coating.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to aqueous, surfactant-stabilized, interpolymer latex binder emulsions that demonstrate excellent dry pick strength when such binder emulsions are incorporated into coating compositions for paper products. Under certain circumstances, the coating compositions can be used as a paper pre-coating, and subsequently overcoated with a top coating composition. Such latex binder emulsions contain an interpolymer comprising a vinyl ester co-monomer which has been copolymerized with a selected amount of ethylene, an unsaturated silane co-monomer, one or more emulsion-stabilizing ionic co-monomers comprising one or more of an unsaturated, substituted sulfonic acid, an unsaturated phosphonic acid, or an ester of one of those acids, and with a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material. The emulsion polymerization can be carried out in conventional manner to form a binder latex containing the desired interpolymer.

The vinyl esters utilized in the formation of the interpolymer of the latex binder emulsions herein are the esters of alkanoic acids, the acid having from one to about 13 carbon atoms. Typical examples include vinyl formate, vinyl acetate, vinyl propionate, vinyl butyrate, vinyl isobutyrate, vinyl valerate, vinyl-2-ethyl-hexanoate, vinyl isooctanoate, vinyl nonate, vinyl decanoate, vinyl pivalate, vinyl versatate, etc. Of the foregoing, vinyl acetate is the preferred monomer because of its ready availability and low cost.

The vinyl ester is preferably present in the interpolymer in an amount ranging from about 67 pphm to 99 pphm (parts per hundred based on total monomers in the interpolymer). More preferably, the vinyl ester content of the interpolyer used in the paper coating compositions herein will range from about 70 pphm to 90 pphm.

The second major component of the interpolymer formed in the binder latex is ethylene. Ethylene will generally comprise from about 1.0 pphm to 30.0 pphm of the interpolymer. More preferably, ethylene will be present in the interpolymer in an amount ranging from about 2 pphm to 25 pphm.

The third component of the interpolymer in the binder latex comprises a minor amount of an unsaturated silane co-monomer. This co-monomer can generally correspond to a substituted silane of the structural Formula I:

in which R denotes an organic radical olefinically unsaturated in the w-position and R¹ R² and R³, which may be identical or different, denote halogen, preferably chlorine, or the group —OZ, Z denoting hydrogen or primary or secondary alkyl or acyl radicals optionally substituted by alkoxy groups.

Suitable unsaturated silane compounds of the Formula I are preferably those in which the radical R in the formula represents an w-unsaturated alkenyl of 2 to 10 carbon atoms, particularly of 2 to 4 carbon atoms, or an w-unsaturated carboxylic acid ester formed from unsaturated carboxylic acids of up to 4 carbon atoms and alcohols carrying the Si group of up to 6 carbon atoms. Suitable radicals R¹, R², R³ are preferably the group —OZ, Z representing primary and/or secondary alkyl radicals of up to 10 carbon atoms, preferably up to 4 carbon atoms, or alkyl radicals substituted by alkoxy groups, preferably of up to 3 carbon atoms, or acyl radicals of up to 6 carbon atoms, preferably of up to 3 carbon atoms, or hydrogen. Most preferred unsaturated silane co-monomers are vinyl trialkoxy silanes.

Examples of preferred silane compounds of the Formula I include vinyltrichlorosilane, vinylmethyldichlorosilane, γ-methacryloxypropyltris(2-methoxyethoxy)silane, vinylmethoxysilane, vinyltrimethoxysilane, vinyltriethoxysilane, vinyldiethoxysilanol, vinylethoxysilanediol, allyltriethoxysilane, vinyltripropoxysilane, vinyltriisopropoxysilane, vinyltributoxysilane, vinyltriacetoxysilane, trimethylglycolvinylsilane, γ-methacryloxypropyltrimethylglycolsilane, γ-acryloxypropyltriethoxysilane and γ-methacryloxypropyltrimethoxysilane.

The unsaturated silane co-monomer as described above is selected and used in forming the interpolymer of the coating compositions used herein in amounts which are effective to alter interpolymer molecular weight, branching and/or flow properties in a certain manner, such as from about 0.1 pphm to about 3 pphm, or even from about 0.1 pphm to about 1.5 pphm, or from about 0.2 pphm to about 0.5 pphm.

The interpolymer additionally contains minor amounts, such as from about 0.1 pphm to about 5 pphm, or from about 0.1 pphm to about 1.0 pphm, or from about 0.2 pphm to about 0.5 pphm of one or more additional co-monomer types that assist in the stabilizing of the latex emulsion which is formed. Such stabilizing co-monomers are those that are ionic in character by virtue of containing acid moieties or the salts or half-esters of such acid moieties. In particular, the interpolymer should contain one or more unsaturated, substituted organic sulfonic acid or salt thereof, unsaturated, organic phosphonic acid or salt thereof, unsaturated, substituted organic sulfonate or sulfate, or unsaturated, organic phosphonate or phosphate. Exemplary monomers of these types are disclosed in U.S. Published Application Nos. 2004/0043155 and 2006/0211594, incorporated herein in their entireties, and include monomers containing at least one sulfate or sulfonate function, such as 2-sulfoethyl methacrylate, 2-acrylamido-2-methylpropanesulfonic acid, styrenesulfonic acid, vinylphosphonic acid, phosphate monomers such as PAM-100 and PAM-200 phosphate ester monomers available from Rhodia and corresponding salts of these monomers, 2-sulfooxyethyl methacrylate, vinylbenzene sulfonic acid, allylsulfonic acid, 2-acrylamido-2-methylpropanesulfonic acid, sulfoethyl acrylate or methacrylate, or sulfopropyl acrylate or methacrylate, and water-soluble salts thereof, acrylamido methyl propane sulfonic acid, styrene sulfonate, sodium vinyl sulfonate, and monomers containing at least one phosphonate or phosphate function, such as vinylphosphonic acid, esters of ethylenically unsaturated phosphates such as phosphates derived from hydroxyethyl methacrylate (Empicryl 6835 from Rhodia) and those derived from polyoxyalkylene methacrylates and water-soluble salts thereof, vinylphosphonic acid, vinyl phosphonate, methacrylic-based phosphonate, or phosphate esters of polyethylene glycol monomethacrylate. The copolymerization with these monomers reduces coagulum and improves the latex runnability in coating color.

Further, the incorporation of these sulfonic or phosphonic acid or ester monomers together with additional acid functionalities, such as those described below, has now been found to surprisingly and unexpectedly improve coating properties such as dry pick resistance.

Other optional ionic monomers include those selected from α,β-ethylenically unsaturated C₃-C₈ monocarboxylic acids, α,β-ethylenically unsaturated C₄ -C₈ dicarboxylic acids and the anhydrides thereof, the C₄-C₈ alkyl half-esters of the α,β-ethylenically unsaturated C₄-C₈ dicarboxylic acids, and acrylic acid and methacrylic acid, and the C₄-C₈ alkyl half esters of maleic acid, maleic anhydride, fumaric acid, and itaconic acid. Acrylic acid is a particularly perferred optional ionic monomer.

The foregoing types of optionally present ionic co-monomers, if employed, are added in very low amounts, for example, in an amount from 0.1 pphm to 5.0 pphm. More preferably, if used, the optional ionic co-monomers will comprise from about 0.2 pphm to about 2.0 pphm, or from about 0.2 to about 1.0 pphm of the monomer mixture.

Optionally, it is possible to prepare interpolymers for use in the paper coating compositions herein which also utilize non-silicon-containing co-monomers in the interpolymers. Such non-silicon-containing co-monomers could be, for example, any unsaturated, multi-functional, cross-linking co-monomers which, when incorporated into the interpolymer in appropriate amounts, provide interpolymers with improved coating strength properties.

Suitable non-silicon-containing unsaturated multi-functional co-monomers can include, for example, unsaturated compounds that contain one or more carbonyl moieties. Preferred co-monomers of this type include those having two or more carbonyl moieties. Examples of such suitable co-monomers include diacetone acrylamide (DiAAA), polymerizable 1,3-dicarbonyl compounds and polymerizable 1,3-diketoamides.

Suitable polymerizable 1,3-dicarbonyl compounds include acetoacetoxyethyl acrylate, acetoacetoxyethyl methacrylate (AEEM), acetoacetoxypropyl methacrylate, acetoacetoxybutyl methacrylate, 2,3-di(acetoacetoxy)propyl methacrylate and allyl acetoacetate.

Suitable polymerizable 1,3-diketoamides include those compounds described in U.S. Pat. No. 5,889,098, which patent is incorporated herein by reference. Examples of compounds of this type include amido acetoacetonates such as 3-isopropenyl-α,α-dimethylbenzyl amidoacetoacetate, 4-isopropenyl-α,α-dimethylbenzyl amidoacetoacetate, 4-ethylenyl-phenyl amidoacetoacetate and the like.

Preferred unsaturated, multi-functional, carbonyl-containing co-monomers of the foregoing types include diacetone acrylamide (DiAAA), acetoacetoxyethyl methacrylate (AEEM), acetoacetoxypropyl methacrylate, acetoacetoxybutyl methacrylate, 2,3-di(acetoacetoxy)propyl methacrylate and allyl acetoacetate. Diacetone acrylamide and acetoacetoxyethyl methacrylate are the most preferred.

Typically unsaturated, carbonyl-functional co-monomers of these types can be present in the polymerization mixture in amounts ranging from about 0.1 to 4.0 pphm. More preferably such co-monomers will be present in amounts from about 0.5 to 3.5 pphm.

In some instances when using such carbonyl functional co-monomers, in order to achieve the desired alteration of interpolymer molecular weight, branching and/or flow properties, it is preferred to also include one or more water-soluble, external cross-linking agents in the emulsion polymerization reaction mixture. Water-soluble external cross-linking agents which can be employed in this manner are those which will react with the carbonyl functionalities of the non-silicon-containing, unsaturated, multifunctional co-monomers which have been polymerized into the interpolymer backbone.

One preferred type of water-soluble cross-linking agent that optionally may be used in the paper coating compositions herein comprises a compound which contains at least two hydrazine moieties. Particularly suitable are dihydrazine compounds of aliphatic dicarboxylic acids of 2 to 10, in particular 4 to 6, carbon atoms, e.g., oxalic acid dihydrazide, malonic acid dihydrazide, succinic acid dihydrazide, glutaric acid dihydrazide, adipic acid dihydrazide, sebacic acid dihydrazide, maleic acid dihydrazide, fumaric acid dihydrazide and/or itaconic acid dihydrazide. Water-soluble aliphatic dihydrazines of 2 to 4 carbon atoms, e.g., ethylene-1,2-dihydrazine, propylene-1,3-dihydrazine or butylene-1,4-dihydrazine, are also suitable. Adipic acid dihydrazide (ADH) is the most preferred water-soluble cross-linking agent for use in the compositions herein.

The water-soluble cross-linking agent will generally be present in the emulsion polymerization reaction mixture in an amount such that the molar ratio of cross-linking agent hydrazine groups to co-monomer carbonyl groups in the blend is between about 0.1 and about 2.0. More preferably the molar ratio of cross-linking agent hydrazine groups to copolymer carbonyl groups in the blend will be between about 0.5 and 1.5. In order to provide such ratios, the dihydrazine water-soluble cross-linking agent can be present in the polymerization mixture in amounts ranging from about 0.1 to 3.0 pphm, more preferably from about 0.2 to 2.0 pphm.

The foregoing unsaturated carboxylic acid-based co-monomers are also generally present in the interpolymers of the binder latex emulsions herein in certain selected amounts which serve to impart the desirable binder strength to paper coating composition containing such emulsions. In particular, this type of unsaturated carboxylic acid based co-monomer will generally be present in the interpolymer in amounts of from about 0.1 pphm to 1.0 pphm. More preferably, such carboxylic acid-based co-monomers will be used in amounts of 0.2 pphm to 0.5 pphm.

Another type of co-monomer that can be used as one of the constituent co-monomers (i.e., a cross-linker) of the interpolymers herein comprises a polyethylenically unsaturated co-monomer selected from the group consisting of triallyl cyanurate, triallyl isocyanurate, diallyl maleate, diallyl fumarate, divinyl benzene and diallyl phthalate. Preferred co-monomers of this type included diallyl maleate, diallyl fumarate and diallyl phthalate. This type of polyethylenically unsaturated co-monomer will be generally present in the interpolymer in amounts of from about 0.05 pphm to 0.5 pphm. More preferably, such polyethylenically unsaturated co-monomer(s)/cross-linker(s) will be used in amounts of from about 0.1pphm to 0.3 pphm.

As noted, the interpolymers used to form the binder latex emulsions of the paper coating compositions herein are made by copolymerizing a vinyl ester with ethylene, an unsaturated silane co-monomer, one or more emulsion-stabilizing ionic co-monomers comprising one or more of an unsaturated, substituted sulfonic acid, an unsaturated phosphonic acid, and/or an ester of one of those acids, and with a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material. Preferably, these several types of co-monomers are present in the interpolymer in amounts relative to each other which serve to impart to the paper coating compositions an IGT Dry Pick Value (as defined hereinafter) of at least about 5% greater than papers coated with conventional coating compositions formulated with styrene-butadiene binders, or even those formulated with VAE latex binders not having the combination of unsaturated silane co-monomer and one or more emulsion-stabilizing ionic co-monomers, described above. Even more preferably, the relative amounts of the co-monomers in the interpolymer are selected such that a paper coating composition containing the interpolymer-based latex emulsion exhibit a Dry Pick Value at least about 10% greater, or even about 15% greater than those more conventional formulations.

The interpolymer comprising the essential co-monomers hereinbefore described can be prepared using conventional emulsion polymerization procedures which result in the preparation of binder latex emulsions for paper coating compositions. Such procedures are described in general, for example, in U.S. Pat. No. 5,849,389, the disclosure of which is incorporated herein by reference in its entirety.

In a typical polymerization procedure, the vinyl ester, ethylene, and other co-monomers can be polymerized in an aqueous medium under pressures not exceeding 100 atmospheres in the presence of a catalyst and at least one emulsifying agent. The aqueous system can be maintained by a suitable buffering agent at a pH of 2 to 6, with the catalyst being added incrementally or continuously. More specifically, vinyl acetate and 50% to 75% of the other co-monomers can be suspended in water and thoroughly agitated in the presence of ethylene under the working pressure to effect solution of the ethylene in the mixture up to the substantial limit of its solubility under the conditions existing in the reaction zone. The vinyl acetate and other-co-monomers can then be gradually heated to polymerization temperature.

The homogenization period is generally followed by a polymerization period during which the catalyst, which preferably comprises a main catalyst or initiator, and may include an activator, is added incrementally or continuously together with the remaining co-monomers. The monomers employed may be added either as pure monomers or as a premixed emulsion.

Suitable polymerization catalysts include the water-soluble free-radical-formers generally used in emulsion polymerization, such as hydrogen peroxide, sodium persulfate, potassium persulfate and ammonium persulfate, as well as tert-butyl hydroperoxide, in amounts of between 0.01% and 3% by weight, preferably 0.01% and 1% by weight based on the total amount of the emulsion. They can be used together with reducing agents such as sodium formaldehyde-sulfoxylate, ferrous salts, sodium dithionite, sodium hydrogen sulfite, sodium sulfite, sodium thiosulfate, as redox catalysts in amounts of 0.01% to 3% by weight, preferably 0.01% to 1% by weight, based on the total amount of the emulsion. The free-radical-formers can be charged in the aqueous emulsifier solution or can be added during the polymerization in doses.

The manner of combining the polymerization ingredients can be by various known monomer feed methods, such as, continuous monomer addition, incremental monomer addition, or addition in a single charge of the entire amounts of monomers. The entire amount of the aqueous medium with polymerization additives can be present in the polymerization vessel before introduction of the monomers, or alternatively, the aqueous medium, or a portion of it, can be added continuously or incrementally during the course of the polymerization.

The emulsion polymerization used to prepare the interpolymer in aqueous latex form is carried out in the presence of a stabilization system which comprises one or more of certain types of anionic and/or nonionic surfactants as emulsifiers. Such emulsifiers are conventional and well known. Suitable nonionic surfactants which can be used as emulsifiers in the emulsion stabilizing system of the coating compositions herein include polyoxyethylene condensates. A wide variety of nonionic surfactants of this type are disclosed in the hereinbefore-referenced U.S. Pat. No. 5,849,389. Suitable anionic surfactants which can be used as emulsifiers in the emulsion stabilizing system of the coating compositions herein include alkyl aryl sulfonates, alkali metal alkyl sulfates, sulfonated alkyl esters and fatty acid soaps. A wide variety of anionic surfactants of this type are also disclosed in the hereinbefore-referenced U.S. Pat. No. 5,849,389.

Conventionally, various protective colloids have been used to stabilize vinyl ester/ethylene emulsion polymer latex compositions of the type hereinbefore described, instead of or in addition to the surfactant emulsifiers. It has been discovered, however, that to realize especially useful compatibility and processing benefits, the coating compositions of the present inventions should be kept substantially free of such protective colloids as polyvinyl alcohol (PVOH), hydroxyethyl cellulose, styrene acrylic acid or styrene maleic acid colloids.

While some very small amounts of protective colloid materials can be tolerated, and may in fact be useful processing the binder emulsion component of the coating compositions used herein, the binder emulsions herein should contain no more than about 1.0 pphm and preferably no more than about 0.5 pphm of protective colloid materials. Likewise, emulsion polymerization can be conducted in the absence of such protective colloid materials. Binder emulsions using surfactant-based stabilizing systems and containing no more than these amounts of protective colloid-forming materials are considered for purposes of this invention to be “substantially free” of protective colloid materials. The latex emulsions which utilize such stabilizing systems are also those characterized herein as being “substantially all-surfactant-based” emulsions.

Following polymerization, the solids content of the resulting aqueous polymer emulsion binder can be adjusted to the level desired by the addition of water or by the removal of water by distillation. Generally, the desired level of polymeric solids content is from about 40 weight percent to about 70 weight percent based on the total weight of the emulsion, more preferably from about 50 weight percent to about 60 weight percent.

If desired, conventional additives may be incorporated into the paper coating compositions used herein in order to modify the properties thereof. Among these additives may be included fillers, thickeners, catalysts, dispersants, colorants, biocides, anti-foaming agents, etc. As indicated above, while it is advantageous to limit or even avoid the use of protective colloids during the emulsion polymerization process, it can be suitable to add similar materials as co-binders to the post-polymerized binder compositions. Such co-binder materials, such as polyvinyl alcohol, can be post-added in amount of at least about 1 wt % of the paper coating composition.

Preferred coating compositions in accordance with the present invention are loaded with filler to yield a composition comprising from about 5 to about 50 weight percent interpolymer, preferably from about 5 to about 20 weight percent, and from about 50 to about 90 weight percent filler, based on total weight of the composition.

Prior to drying, the paper coating compositions herein can generally have a total solids content ranging from about 50 wt % to 95 wt %. More preferably, prior to drying, the coating compositions herein will have a total solids content ranging from about 80 wt % to 90 wt %.

The particle size of the latex can be regulated by the quantity of non-ionic or anionic emulsifying agent or agents employed. To obtain smaller particles sizes, greater amounts of emulsifying agents are used. As a general rule, the greater the amount of the emulsifying agent employed, the smaller the average particle size.

The actual paper coating compositions herein comprise the interpolymer latex together with a pigment, such as clay and/or calcium carbonate, and the usual paper coating additives which may include other co-binders, such as polyvinyl alcohol, protein, e.g. casein or soy protein, or starch, as is well known to those skilled in the art. The coating compositions herein will also contain sufficient alkali to maintain the pH of the coating composition between 6 and 10, more preferably between 7 and 9.

The pigment used in the paper coating compositions herein may be any of those conventionally employed. Frequently, some or all of the pigment comprises clay and for this portion any of the clays customarily used for paper coating, including the hydrous aluminium silicates of kaolin group clays, hydrated silica clays, and the specific types of clays recommended in Chapters 10-16 of “Kaolin Clays and their Industrial Uses,” by J. M. Huber Corp. (1949), New York, N.Y.

In addition to clay itself, or as a complete replacement for clay, there may also be utilized other paper pigments such as, for example, calcium carbonate, titanium dioxide, blanc fixe, lithopone, zinc sulfide, or other coating pigments including plastics, for example polystyrene. When used with clay, the other pigments can be present in various ratios, e.g. up to 50%, preferably up to 35%, by weight of the clay. Alternatively, the pigment can comprise more than 50% of calcium carbonate relative to clay, even more than 75% calcium carbonate relative to clay, or even 100% calcium carbonate in the absence of clay. Additionally, the composition may also contain other additives such as zinc oxide and/or a small amount, of a dispersing or stabilizing agent such as tetrasodium pyrophosphate.

In general, the paper coating compositions herein can comprise 100 parts pigment, e.g., pigments which comprise 65-100 parts calcium carbonate and 0-35 parts secondary pigment; 0.01-0.5 parts dispersing or stabilizing agent; 3-30 parts interpolymer latex (solids basis); 0-25 parts cobinder; optionally 0.02 parts defoamer; and sufficient water to provide the desired level of solids. Coating compositions containing from about 40 wt % to 70 wt % solids are typical. The modification and formulation of the coating color using these materials will be within the knowledge of those skilled in the art.

The paper coating compositions herein may be applied to various planar, fibrous cellulose substrates including paper such as freesheet and groundwood grades; paper board; labels; paper products used for newspapers, advertisements, poster, books or magazines; and building substrates such as wall paper, wall board, or ceiling tile. In one embodiment, the paper coating composition can be used to coat paper intended for rotogravure printing.

The amount of the paper coating composition applied to the planar fibrous cellulose substrate is generally in the range of about 1 g/m² to about 30 g/m², and preferably in the range of about 3 g/m² to about 25 g/m², or from about 10 g/m² to about 20 g/m². The paper coating composition may be applied in a single step or by using two or more steps to build the final coat weight. Further, the paper coating composition may also be applied to the second side of the substrate either simultaneously or as a separate coating step.

The paper coating composition may be applied to the substrate by techniques well known to those in the art. For example, the paper coating composition may be applied with a roll applicator such as a metered size press; a blade coater such as a short dwell time applicator; air knife coater; slot die coater such as a jet applicator; or brush. Preferred coating methods for high speed application include the use of a blade coater or a metered size press.

In one embodiment, the invention is also directed to a process for forming a coated paper product, comprising (a) coating a planar fibrous cellulose substrate with the coating composition described above to form a wet coated substrate; (b) drying the wet coated substrate to form a dried coated substrate; and (c) calendering the dried coated substrate to form the coated paper product.

The paper coating compositions of the present invention, which contain the particular vinyl ester-based binder latex emulsions described herein, provide improved binding strength when applied as coating to paper substrates of the type described above. This improved binding performance can be quantifed by means of a parameter called dry pick strength, and in particular by means of a parameter called Dry Pick Value as specifically defined hereinafter.

Picking is defined as the lifting of a coating, film or fibers from the surface of the base paper during printing. When a print wheel makes contact with a paper sample to deposit the ink, then subsequent negative forces are exerted on the paper as the inked print wheel is removed from the paper surface. The dry pick strength of the coated paper is measured with a method that consists of printing a strip of the coated paper in a print tester at an accelerating rate. The accelerated speed of the print wheel and the tack rating of the ink are adjusted to determine the strength of the coated paper sample at specific printing conditions. If the combination of print wheel speed and ink tack is great enough, then resulting negative forces create picking, which may appear as: white areas on the surfaces of the print wheel and coated paper sample, as blisters and textured areas on the surface of the coated paper sample, as delamination (surface layer removal) of the coated paper sample, or as tearing (complete strength failure) of the base paper sample.

Evaluation of the picking effect exhibited by selected paper substrates coated with any given type of paper coating composition can be used to quantify the binding strength and coating performance of that composition. Picking evalution is carried out by means of IGT pick testing according to standard methods of measurement by the Technical Association of the Pulp and Paper Industry (TAPPI) as well known in the art. A measure of dry and wet binding strength is provided by IGT Pick testing pursuant to TAPPI Useful Method UM 591, Surface Strength of Paper. The IGT dry pick strength measures the speed, in cm/sec, required to lift the paper coating off of the surface of a paper substrate strip when printed using an ink roller and standard conditions as described in UM 591. Higher IGT dry pick numbers indicate better resistance of the coated substrate to picking and hence higher strength coating performance.

Generally, the binder level in the formulations of the present invention were selected to emulate commercial coating recipes. Wire wound rods were used to coat a bleached substrate at a target coating weight value of 12 lbs./3000 ft² (19.5 g/m²). This coating weight was selected to mimic the bleached board market. The freshly coated boards were oven dried at about 260° F. (127° C.) for 30 seconds and subsequently calendered at 600 psi (4.14 MPa) and 170° F. (77° C.) using 1 nip. The finished boards were allowed to sit for 24 hours under constant temperature and humidity conditions (72° F. (22° C.), 50% RH) before being tested for IGT pick resistance.

The Dry Pick Values as used herein are the values obtained from the dry pick testing of the coated boards as described above using an IGT Testing Systems AIC2-5 Printability Tester under conditions which include use of medium viscosity oil, 2 cm/sec, and 50 KgF. As noted above, the paper coating compositions of the present invention, when using the binder emulsions of the present invention, exhibit Dry Pick Values in accordance with the above-described testing of at least about 75, more preferably at least about 90, and even more preferably of at least about 100. Generally, the paper coating compositions of the present invention exhibit Dry Pick Values of from about 90 to 110.

The ink wettability or receptivity of a test coating is an important characteristic of a printing grade paper. A common industry test of ink receptivity is K&N Ink Brightness, described in TAPPI Test Method RC19. The K&N ink test consists of an oil-soluble dye in a varnish-base ink applied in excess to a 1-2 square inch area of test paper. The ink receptivity is measured by the percentage surface brightness drop when K&N ink is applied for a short time and removed. The smaller the number for ink receptivity, the more nonporous the coating is and therefore a lesser degree of ink penetration into the coating. Low numbers indicate greater ink receptivity.

EXAMPLES

A number of binder emulsions based on vinyl acetate/ethylene (VAE) interpolymers were prepared using the general polymerization techniques described in Comparative Example 1 and in Examples 1-4 of U.S. Pat. No. 5,576,384, incorporated herein by reference. The interpolymers in the emulsions made contain relatively small amounts of additional co-monomers including some of the ionic emulsion stabilizing co-monomers and the unsaturated silane co-monomers used in the present invention. All of the binder emulsions made were emulsified with various amounts of anionic and/or nonionic emulsifiers. Some of the binder emulsions made also contain varying amounts of protective colloids based on polyvinyl alcohol (PVOH), hydroxyethyl cellulose, styrene acrylic acid or styrene maleic acid colloids.

The components of each of the various binder emulsions made are described in Table 1.

TABLE 1 BINDER EMULSIONS Exam- ple Main Functional No. Type Monomers Monomers Surfactants Colloid 1 VAE 91.2VA/ SVS-0.2 Anionic-0.5 PVOH-0.5 8.8E VTMOsilane-0.4 Nonionic-3 2 VAE 91VA/9E SVS-0.2 Anionic-3.0 PVOH-0.5 VTMOsilane-0.4 3 VAE 91.2VA/ SVS-0.2 Anionic-0.5 PVOH-0.5 8.8E VTMOsilane-0.4 Nonionic-3 4 VAE 87VA/13E SVS-0.5 Anionic-0.5 None VTMOsilane-0.5 Nonionic-3 5 VAE 87VA/13E SVS-0.5 Anionic-0.5 PVOH-0.2 VTMOsilane-0.5 Nonionic-3 6 VAE 87VA/13E SVS-0.5 Anionic-0.5 PVOH-0.2 VTMOsilane-0.5 Nonionic-3 AA-1.0 C.E. 1 VAE 87VA/13E SVS-0.5 Anionic-0.5 PVOH-0.2 Nonionic-3

In Table 1, the following designations are used:

-   -   VA is vinyl acetate, E is ethylene.     -   VAE represents a vinyl acetate/ethylene binder emulsion.     -   VTMOsilane is vinyl trimethoxy silane.     -   SVS is sodium vinyl sulfonate and AA is acrylic acid.

PVOH is polyvinyl alcohol and HEC is hydroxyethyl cellulose.

-   -   SBR is styrene-butadiene rubber.     -   Numerical values are parts per hundred based on total of VA and         E co-monomers.

Dry Pick Value Determination

The binder latex emulsion of Example 6 was formulated into a pre-coating composition as described in Table 2 below, and was compared to a conventional pre-coating composition containing commercially obtained SBR as the binder polymer.

TABLE 2 PRE-COATING FORMULATIONS Conventional Binder Example 6 Binder Components Dry parts Dry parts C60* 100 100 dispersant 0 0.25 antifoamer 0.02 0.02 SBR 8.75 0 VAE 0 8.75 Starch 3.75 3.75 CMC 0.5 0.5 Glyxoal 0.6 0.8 lubricant 0.6 0.6 biocide 0.01 0.01 *C60 represents Hydrocarb 60, calcium carbonate

The pre-coating compositions of Table 2 were coated onto a paper substrate at levels of 22 g/m², calendered, dried in an oven, and overcoated with a conventional top coating composition containing SBR, formulated as in Table 3 below, at a level of 22 g/m².

TABLE 3 SBR TOP COATING FORMULATION Top Coat Components Dry parts C98* 90 clay 10 dispersant 0.06 antifoamer 0.02 soda 0.6 SBR 11.5 CMC 0.15 rheology modifier 0.35 Glyxoal 0.5 lubricant 0.5 biocide 0.01 *C98 represents Hydrocarb 98, calcium carbonate

The coated paper was again calendered, dried in an oven and tested for IGT dry pick resistance and other relevant parameters. The results are set forth in Table 4 below.

TABLE 4 Parameter Conventional SBR Pre-coat Example 6 Pre-coat IGT dry pick (cm/s) 89 123 K&N ink pickup (%) 20.8 21.4 Sheet gloss (%) 63.5 64 Brightness (%) 83.5 83.5

The testing results in Table 4 indicate that coated paper having the pre-coating compositions formulated with the latex binder interpolymers according to the present invention provide paper coatings with especially desirable IGT Dry Pick performance.

Three paper coating compositions were formulated as set forth below:

70 dry parts Hydrocarb 60 (CaCO3)

30 dry parts Ultrawhite 90 (clay)

0.1 dry parts Finnfix 10G (CMC)

0.1 dry parts Dispex N40 (dispersant)

17 dry parts latex (binder).

The total solids contents of the coating compositions were targetted at 64%, and three different binders were used: a commercially available SBR, the latex binder of Example 5 and the latex binder of Example 6 (Table 1). The coating compositions were adjusted to a pH between 8.0 and 9.0 with ammonium hydroxide. The resulting compositions were coated onto SBS boards at coating weights of 19.5 g/m², calendered and dried as described above. FIG. 1 illustrates the comparative IGT Dry Pick Resistance between the three coated boards. The results of the testing are set forth in Table 5, below.

TABLE 5 IGT Dry Coating Sample Strength Brightness Whiteness b-Value Gloss SBR Binder 409 85.7 92.3 1.5 35.4 Example 6 latex 364 86.6 92.6 1.2 36.4 Example 5 latex 305 87.2 92.9 1.4 33.7

As can be seen from the tabular data, the latex binder of Example 6 provided an unusually and unexpectedly high IGT Dry Coating Strength, even as compared to that of Example 5. The IGT Dry Coating Strength of boards coated with the coating composition containing the Example 6 latex binder is more than 15% greater, in fact nearly 20% greater than that containing the Example 5 latex binder.

While the present invention has been described and illustrated by reference to particular embodiments, those of ordinary skill in the art will appreciate that the invention lends itself to variations not necessarily illustrated herein. For this reason, then, reference should be made solely to the appended claims for purposes of determining the true scope of the present invention. 

We claim:
 1. A paper product comprising: a planar fibrous cellulose substrate; and a coating composition disposed on a surface of said substrate comprising an interpolymer prepared by the emulsion polymerization of: i) one or more vinyl ester monomers; ii) ethylene; iii) an unsaturated silane co-monomer; and iv) one or more emulsion-stabilizing ionic co-monomers comprising one or more of: (a) an unsaturated, substituted organic sulfonic acid or salt thereof, (b) an unsaturated, organic phosphonic acid or salt thereof, (c) an unsaturated, substituted organic sulfonate or sulfate, or (d) an unsaturated, organic phosphonate or phosphate; in the presence of: v) a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material; and vi) water.
 2. The paper product according to claim 1 wherein the interpolymer comprises from about 0.1 pphm to about 5 pphm of said one or more emulsion-stabilizing ionic co-monomers.
 3. The paper product according to claim 1 wherein the one or more emulsion-stabilizing ionic co-monomers is selected from the group consisting of acrylamido methyl propane sulfonic acid, styrene sulfonate, and sodium vinyl sulfonate.
 4. The paper product according to claim 1 wherein the one or more emulsion-stabilizing ionic co-monomers is selected from the group consisting of vinylphosphonic acid, vinyl phosphonate, methacrylic-based phosphonate, phosphate esters of polyethylene glycol monomethacrylate, and (meth)acrylic phosphates.
 5. The paper product according to claim 1 wherein the emulsion-stabilizing ionic co-monomers further comprise one or more of acrylic acid, methacrylic acid, and the C₄-C₈ alkyl half esters of maleic acid, maleic anhydride, fumaric acid, and itaconic acid.
 6. The paper product according to claim 1 further comprising about 2 pphm of acrylic acid co-monomer.
 7. The paper product according to claim 1 wherein the interpolymer comprises from about 0.1 pphm to about 3 pphm of said unsaturated silane co-monomer.
 8. The paper product according to claim 1 wherein the vinyl ester monomer is vinyl acetate and the unsaturated silane co-monomer is a vinyl trialkoxy silane.
 9. The paper product according to claim 8 wherein the unsaturated silane co-monomer is selected from the group consisting of vinyltrimethoxysilane, vinyltriethoxysilane, ω-methacryloxypropyl-trimethoxysilane, vinyl tris(2-methoxyethoxy)silane, vinyltriisopropoxy silane, vinylpropyltriisopropoxy silane, vinylpropyltriisobutoxy silane, vinyltriisobutoxy silane, vinylpentyltri-t-butoxy silane, vinylpropyl-methyldipentoxy silane, and vinylpropyltri-sec-butoxysilane.
 10. The paper product according to claim 1 wherein the stabilizing system comprises polyoxyethylene condensates.
 11. The paper product according to claim 1 wherein the stabilizing system comprises anionic surfactants selected from the group consisting of alkyl aryl sulfonates, alkali metal alkyl sulfates, sulfonated alkyl esters and fatty acid soaps.
 12. The paper product according to claim 1 wherein the interpolymer component is prepared by emulsion polymerizing: i) from about 67 to about 99 pphm of the vinyl ester monomer; ii) from about 1.0 to about 30.0 pphm of ethylene; iii) from about 0.1 to about 3.0 pphm of the unsaturated silane co-monomer; and (iv) from about 0.1 to about 1 pphm of at least one of the unsaturated, substituted sulfonic acid, the unsaturated phosphonic acid, or an ester or salt thereof.
 13. The paper product according to claim 1 wherein the coating composition further contains from about 50 wt % to about 90 wt % of filler material selected from the group consisting of calcium carbonate, clay, titanium dioxide, blanc fixe, lithopone, zinc sulfide, polystyrene and combinations thereof.
 14. The paper product according to claim 13 wherein the coating composition comprises more than about 50 wt % calcium carbonate as the filler material.
 15. The paper product according to claim 1 wherein the coating composition has a total solids content of from about 50% to about 95% by weight prior to drying.
 16. The paper product according to claim 1 wherein the emulsion polymerization contains less than about 0.5 pphm of a protective colloid material selected from the group consisting of polyvinyl alcohol, hydroxyethyl cellulose, styrene acrylic acid, styrene maleic acid and carboxymethyl cellulose.
 17. The paper product according to claim 15 comprising from about 5 wt % to about 20 wt % of said interpolymer.
 18. The paper product according to claim 1, further comprising a top coating composition disposed on said coating composition.
 19. The paper product according to claim 18, wherein the coating composition is provided in an amount effective to bind the top coating composition to said fibrous cellulose substrate with an IGT dry pick strength at least about 5% greater than a coating composition containing SBR binder, or VAE binder without said co-monomers.
 20. The paper product according to claim 1, further comprising at least about 1 wt % polyvinyl alcohol, post-added to the composition as a co-binder.
 21. A process for forming a coated paper product, comprising: (a) coating a planar fibrous cellulose substrate with a coating composition to form a wet coated substrate, wherein the coating composition comprises an interpolymer prepared by the emulsion polymerization of: i) one or more vinyl ester monomers; ii) ethylene; iii) an unsaturated silane co-monomer; and iv) one or more emulsion-stabilizing ionic co-monomers comprising at least an unsaturated, substituted organic sulfonic acid or salt thereof, an unsaturated, organic phosphonic acid or salt thereof, an unsaturated, substituted organic sulfonate or sulfate, or an unsaturated, organic phosphonate or phosphate; in the presence of: vi) a stabilizing system which comprises one or more anionic and/or nonionic surfactants but less than about 1 pphm of any protective colloid material; and vii) water; (b) drying the wet coated substrate to form a dried coated substrate; and (c) calendering the dried coated substrate to form the coated paper product. 